The present invention relates to optical systems for converting electromagnetic radiation modes. More particularly, the present invention relates to optical systems for converting gyrotron waveguide output modes into free-space radiation modes suitable for multi-mirror or waveguide transmission. The present invention is probably best compared with a Vlasov antenna mode converter (S. N. Vlasov et al., Radio Eng. Electron. Phys. 20, 14, 1975), however it has higher efficiency than the Vlasov design, and its greater symmetry enables simpler performance analysis.
The primary application of the high-power, high-frequency gyrotrons developed over the last decade is in the electron cyclotron resonance heating (ECRH) of fusion plasmas. With ECRH the microwave energy generated by a gyrotron is channeled or transported to a plasma containment vessel via a waveguide or mirror system. However, one problem associated with increasing the output power and frequency of resonant-cavity gyrotrons is that of mode competition within the cavity. To alleviate this problem a class of rotating surface modes called "whispering-gallery" modes are used for operation because they have a relatively low mode competition, and the corresponding whispering-gallery output is in a well defined mode. Transverse electric Bessel function solutions TE.sub.m,n, for which m&gt;&gt;n, describe the whispering-gallery modes. (Here m and n are the integers associated with the cylindrical variables .phi. and r, respectively.) Unfortunately, the whispering-gallery modes are not suitable for lengthy transmission from a gyrotron to a containment vessel.
Therefore there is a need to convert the whispering-gallery mode output of a gyrotron into a more suitable mode for practical applications requiring long distance transmission of the gyrotron power to the location it is used, e.g. to a plasma containment vessel. Long distance, low loss transmission may be accomplished with either a multi-mirror system or via a waveguide employing a suitable waveguide mode. A linearly polarized, Gaussian-like beam is suitable for coupling power to either mode of transmission.